Russian military sources are quoted as saying that a group of soldiers captured in eastern Ukraine had crossed the border “accidentally”.
The RIA Novosti news agency cites a Russian Defence Ministry source as saying the troops were part of a border patrol, and surrendered without resistance.
Ukraine has released video interviews with some of the 10 soldiers it says were detained about 20 kilometres into its territory.
A soldier identified as Ivan Milchakov said: “We were on the march from the Rostov region (in Russia), from the village of Matveyev Kurgan, on a 70-kilometre march to Ukraine.”
“Did you know that you were on Ukrainian territory illegally?” the interviewer asks him.
“I suspected so, but I became sure when they started bombing us,” the soldier replies.
Ukraine dismissed the Russian claims that the soldiers crossed the border by accident, arguing that they were on a special mission.
Moscow has denied accusations by Kyiv and the West that it has been supporting pro-Russian separatists with arms and troops.
On Monday (25 August) Ukraine said it stopped a convoy of Russian armoured vehicles that had crossed the border.